Causes of low Compression

[SeanNJ]

Fidiro, sorry to hear the 029 is going back to the work bench. That is frustrating. You're going to be a pro with these saws.

I have some good news to report. My 029 is running pretty good now. All I did was mix up a fresh batch of 50:1 fuel. I doesn't cut too good because the chain is dull so I stopped cutting before I strained it.

On Friday I mixed up a batch of 40:1 fuel and it didn't like it. It kept stalling.

I'm going to do another compression test to see if anything changed.


A. Palmer, that's good news that your piston and rings are good. Probably means your cylinder is good too.

Try checking the piston and rings from the carb port. A little harder to do since you will have to remove the carb. Try poking the rings to see if they're stuck in the piston or if they move.

Another thing you try is to slowly pull the starter rope while you have the muffler or carb off and listen for air leaking past the rings. Sometimes it helps to put a couple drops of premix oil down the spark plug hole so you can hear the hiss or even see little bubles around the rings. If it's a lot of air leaking, it may be that the rings are worn or too weak to hold good compression.

It may also be an end gap issue like Fidiro. The end gap is where the ends of the ring meet.

Here's some helpful videos if you decide to start tearing it down:

YouTube - ‪Stihl MS250 Chainsaw Teardown‬‏

YouTube - ‪Repair of Stihl MS250 Chainsaw Part 1 of 4‬‏

Hope this helps,
Sean

 

[a. palmer jr.]

Fidiro, sorry to hear the 029 is going back to the work bench. That is frustrating. You're going to be a pro with these saws.

I have some good news to report. My 029 is running pretty good now. All I did was mix up a fresh batch of 50:1 fuel. I doesn't cut too good because the chain is dull so I stopped cutting before I strained it.

On Friday I mixed up a batch of 40:1 fuel and it didn't like it. It kept stalling.

I'm going to do another compression test to see if anything changed.


A. Palmer, that's good news that your piston and rings are good. Probably means your cylinder is good too.

Try checking the piston and rings from the carb port. A little harder to do since you will have to remove the carb. Try poking the rings to see if they're stuck in the piston or if they move.

Another thing you try is to slowly pull the starter rope while you have the muffler or carb off and listen for air leaking past the rings. Sometimes it helps to put a couple drops of premix oil down the spark plug hole so you can hear the hiss or even see little bubles around the rings. If it's a lot of air leaking, it may be that the rings are worn or too weak to hold good compression.

It may also be an end gap issue like Fidiro. The end gap is where the ends of the ring meet.

Here's some helpful videos if you decide to start tearing it down:

YouTube - ‪Stihl MS250 Chainsaw Teardown‬‏

YouTube - ‪Repair of Stihl MS250 Chainsaw Part 1 of 4‬‏

Hope this helps,
Sean

Sean, that was a lot of help, still seems like a lot of tearing down just to get to the piston and bearings. I guess I'm used to the older saws.

 

[SeanNJ]

I'm not familiar with working on the 021 model but if it's anything like the 029, it's a bit more work. The pro models are easier to open up.

This reason alone has caused me to rethink my next model saw if I ever buy one. I had no idea they did this clam shell design on my 029 until I had to open it up. I'm not opposed to rebuilding a saw engine now and then. But please don't make it more difficult for us. Ha! But I understand it's their consumer line and I'm sure they weren't factoring in saw owners rebuilding their own saws like we're doing.

 

[a. palmer jr.]

I'm not familiar with working on the 021 model but if it's anything like the 029, it's a bit more work. The pro models are easier to open up.

This reason alone has caused me to rethink my next model saw if I ever buy one. I had no idea they did this clam shell design on my 029 until I had to open it up. I'm not opposed to rebuilding a saw engine now and then. But please don't make it more difficult for us. Ha! But I understand it's their consumer line and I'm sure they weren't factoring in saw owners rebuilding their own saws like we're doing.

Some saw owners don't...they give them a pitch instead, right into the dumpster where most "homeowner" saws belong! The fellow that owned this saw before me said he was getting ready to take it to the dump if someone didn't buy it pretty soon.